1. Field of the Invention
The methods and compositions described below are useful for conditioning and removing solid deposits that typically incorporate a mixture of metallic and semimetallic oxides including, for example, scale deposits formed on surfaces within a steam generating system. The methods and compositions are not, however, limited to scale and will also be useful for removing deposits incorporating a wide range of mixtures of metallic and semimetallic compounds including, for example, anhydrous or hydrated oxides and/or hydroxides singly or in combination with nitrates, sulfates, carbonates and/or phosphates that have accumulated within tubing, pipes, vessels and/or other components. The particular mix of compounds present within any particular deposit depends on a number of factors including, for example, the source water composition, treatment chemistries added to the source water, the composition of the components and the conditions under which the system is operated.
2. Description of the Background Art
It is well known that various impurities, introduced into steam generating systems, give rise to solid deposits that form on the surface of components of such systems particularly including components involved in higher temperature operations including, for example, heat exchangers, steam generators and turbines. The presence of such solids, typically including a mixture of metallic and semimetallic compounds as noted above are variously described as scale, deposits, or sludge depending on their characteristics and location within the steam generating system. Despite efforts to reduce such deposits by controlling the cycles of concentration within the systems and chemical additives incorporated in the circulating fluid, scale and other deposits continue to be a concern in most, if not all, steam generating systems.
The particular terms used to the describe the deposits notwithstanding, the accumulation of such compounds on surfaces and within vessels may have various adverse effects on steam generating system operation, including: (1) decreasing heat transfer to the secondary coolant within the steam generator, resulting in loss of heat exchange efficiency, (2) clogging or partial clogging of flow passages in tube supports or other internal steam generator structures, (3) promoting under-deposit corrosion, which leads to accelerated local corrosion of the affected surfaces such as the tubes in a shell-and-tube steam generator, and (4) imparting high stresses on steam generator components. The deposit-induced stresses can result in mechanical deformation and cracking of steam generating equipment components.
Thus, removing such deposits through chemical or mechanical means is desirable and typically achieved through periodic cleaning operations to remove deposits in order to reduce the accumulation of deposits on surfaces of components of the steam generating system. As an alternative to complete removal of deposits, deposits can be treated by conditioning processes. Conditioning of scale, deposits or sludge assists in their removal and fluidization, which is beneficial. Such conditioning may involve softening, partial dissolution, formation of pores, detachment of the solids from the surfaces, or any combination thereof.
Solids deposited in steam generating systems commonly contain primarily iron oxides such as magnetite as a result of typical materials of construction used in steam generating systems and auxiliary systems. However, solids deposited in different parts of steam generating systems may have different compositions. For instance, solids deposited in the lower bundle region of steam generators often have a high content of oxides and hydrated oxides of aluminum and silicon, relative to those present in the upper bundle region. Such oxides and hydrated oxides may include, for instance, boehmite (AlOOH) and silica (SiO2). Deposits containing oxides of aluminum and silicon are also frequently encountered in boilers in fossil fuel plants. Oxides and hydrated oxides of aluminum and silicon tend to act as binding species that consolidate deposits throughout the steam generating system. Thus, deposits containing these species are generally more difficult to dissolve and remove than other common solids found in steam generator deposits, such as magnetite or copper.
Nuclear waste sludge such as that found in long term storage facilities in both the U.S. and internationally, which may receive and accumulate compounds from various processes, can be even more complex and include a mixture of aluminum, sodium, iron, calcium, manganese, bismuth, uranium, silver, copper, zirconium and lanthanum compounds. Representative compounds identified in nuclear waste sludge have included, for example, Al(OH)3, Gibbsite; (NaAlSiO4)6.(NaNO3)1.6.2H2O, NO3—Cancrinite; AlO(OH), Boehmite; NaAl(CO3)(OH)2, Dawsonite; Fe2O3, Hemtatite; Ca5OH(PO4)3, Hydroxylapatite; Na2U2O7, Clarkeite; ZrO2, Baddeleyite; Bi2O3, Bismite; SiO2, Quartz-Silica; Ni(OH)2, Theophrasite; MnO2, Pyrolusite; CaF2, Fluorite; LaPO4.2H2O; Ag2CO3 and PuO2.
Deposits rich in magnetite and copper, such as those found throughout the steam generators in pressurized water reactor (PWR) nuclear power plants, can be effectively removed using solvents with a high concentration of EDTA, accompanied by hydrazine at near neutral pH (˜7) for iron removal or by hydrogen peroxide at weakly basic pH (˜9.5) for copper removal. Schneidmiller, D. and Stiteler, D., Steam Generator Chemical Cleaning Process Development, EPRI, Palo Alto, Calif., EPRI NP-3009 (1983). However, such solvents are much less effective in the removal of deposits rich in aluminum oxides and silicon oxides, which are typically found at or near tube to tubesheet intersections in a vertically-oriented PWR steam generator, but may also be found in other locations in steam generating systems. (The tubesheet is the bottom surface of the secondary (boiling) side of a vertically-oriented steam generator.)
Generally, two types of cleaning operations are used to remove accumulated deposits from steam generating systems. One type of cleaning operation involves the use of chemical solutions with high concentrations, typically from about 2 to about 15 wt % or more, of solutes. Severa, J. and Bár, J., Handbook of Radioactive Contamination and Decontamination, Elsevier, Amsterdam, 1991. Those skilled in the art will appreciate that while the concentration of the solute used for such processes is, for convenience, typically expressed in terms of wt %, it is well understood that the capacity of the chemical solution is actually a function of the molar concentration of the solute. Such concentrated chemical cleaning methods require extensive time to prepare the temporary equipment system used to implement the cleaning operation, and the required use and disposal of large quantities of chemicals renders the use of such methods very costly.
In contrast, a second type of cleaning operation makes use of solutions at much lower concentrations, typically less than about 0.1 wt % (approximately 1000 ppm), but often up to or slightly above 1 wt % (approximately 10,000 ppm). Such dilute chemical cleaning methods do not require adaptation of large temporary equipment systems to the existing steam generating system to be cleaned, thus making it possible to implement such cleaning processes within a short period, often with little or no impact on other activities planned during regularly-scheduled maintenance outages. In addition, such methods do not require large amounts of chemicals. Accordingly, cleaning operations of this type are much less complicated and much less expensive than more concentrated chemical cleaning methods. Examples of several dilute cleaning methods are discussed below.
Fellers' U.S. Pat. No. 5,779,814 (“Fellers I”) and U.S. Pat. No. 6,017,399 (“Fellers II”) disclose methods for controlling and removing solid deposits from surfaces of components of a steam generating system by adding to the aqueous phase of the steam generating system one or more volatile amines having a pKa value greater than about 10.61 at 25° C. Such amines were selected from the group consisting of alkyl amines, cycloalkyl amines, and primary, secondary, and tertiary amine derivatives. Dimethylamine (pKa of about 10.61 at 25° C.) is a most preferable member of the group. Pyrrolidine, a cycloalkyl amine with a pKa of about 11.27 at 25° C., is also highly preferred. Other volatile amines which are mentioned in the invention range from mono-N-butylamine (MBNA) with a pH of 10.61 at 25° C. to 1,5-diazabicyclo(5,4,0)undec-5-ene with a pH of 13.40 at 25° C. The concentration of the amine applied was from about 0.01 ppm to 50 ppm, preferably from about 0.5 ppm to 50 ppm, most preferably from about 0.5 ppm to 10 ppm. This method discloses the addition of such amines to both the aqueous phase used to generate steam during on-line continuous operation of the steam generating system and to an aqueous phase present in the steam generating system when the system is shut down. In practice, such amines have been added to layup solutions present in steam generators during regularly-scheduled maintenance outages in order to promote removal of deposit constituents such as copper and lead. Marks, C., Lead Risk Minimization Program at Palisades Generating Plant, EPRI, Palo Alto, Calif., EPRI 1016556 (2008) (“the Marks article”); Stevens, J., et al., “Steam Generator Deposit Control Program Assessment at Comanche Peak”, Chemie 2002 Proceedings: International Conference Water Chemistry in Nuclear Reactors Systems Operation Optimization and New Developments Volume 3. Avignon, France, Apr. 22-26, 2002 (“Stevens”); Fellers, B., and J. Wooten, “Alternative Amines Improve Plant Performance at Comanche Peak Steam Electric Station”, Presented at EPRI Nuclear Plant Performance Improvement Seminar, Charleston, S.C., Aug. 3-4, 1994 (“B. Fellers”). Such amines have also been added to the secondary system during power operation at concentrations ranging from several ppb to several ppm as a means of controlling the pH within a specified band. Effects of Different pH Control Agents on Pressurized Water Reactor Plant Systems and Components, EPRI, Palo Alto, Calif.: 2007. 1019042.
Rootham's U.S. Pat. No. 5,764,717 (“Rootham I”) and Rootham et al.'s U.S. Pat. No. 5,841,826 (“Rootham II”) disclose the use of an aqueous cleaning solution comprising a cleaning agent from at least one of the group consisting of a carrier agent and an intercalation agent, or a combination thereof, wherein said carrier agent is selected from the group consisting of dimethylamine, ethylamine, 1,2-diaminomethane, diaminopropane, ethanolamine, 2-methyl-2-amino-1-propanol, 5-aminopentanol, and methoxypropylamine, where the cleaning agent is provided in a concentration of less than 0.1 wt % of said solution. The method further comprises the use of pressurized pulses within said cleaning solution to dislodge and fluidize sludge and deposits accumulated in a heat exchange vessel.
Rootham et al.'s U.S. Pat. No. 6,740,168 (“Rootham III”) discloses a method of conditioning and removing scale and deposits within a heat exchange system, said scale conditioning agent comprising a chelant (such as EDTA, HEDTA, lauryl substituted EDTA and/or an organic acid such as oxalic acid, citric acid, maleic acid or mixtures thereof), a reducing agent (such as ascorbic acid, isomers of ascorbic acid, citric acid, hydrazine, catalyzed hydrazine, or carbohydrazide), a pH control agent, in particular a nitrogen containing aliphatic compound having fewer than 10 carbons such as triethanolamine, dimethylamine, ethylamine, 1,2-diaminoethane, diaminopropane, ethanolamine, diethanolamine, 2-methyl-2-amino-1-propanol, 5-aminopentanol, or methoxypropylamine, and a non-ionic surfactant such as Triton X-100. The treatment concentration of this scale conditioning agent in the aqueous cleaning solution is less than 1 wt %, the treatment temperature is less than 100° C., and the treatment pH is from 3.5 to 9.